Rupert Howell, McCann's UK and Ireland chairman and its president, EMEA, is set to take on Warden's duties.
Warden, 57, who joined McCann in 1992, is understood to have visited its New York HQ last week to discuss exit plans.
Warden's first McCann role was heading its New York office. He returned to his native UK as the chairman and chief executive of McCann UK Group in 1994. He was also the regional director for Central and Eastern Europe.
Howell joined McCann in October 2003 with a brief to turn around the London shop, in addition to his European responsibilities. London took up most of his time in the first 18 months; however, this year he has turned his attention to developing Central and Eastern European operations.
A McCann spokesman did not deny Warden was retiring, but said news of his exit was premature. "When Howell joined he had a European title and he is an important part of a succession plan," he said.
Warden was not available for comment.
- Comment, page 36.